It is known that millimeter waves penetrate numerous adverse environments which are not transparent to optical or infrared rays. For example, millimeter waves penetrate fog, snow, blowing dust and sand, smoke, clouds of chemical gases, and other atmospheric constituents which absorb visible and infrared radiation, and therefore prevent image formation using systems operated at these wavelengths. For example, fog currently prevents landing of airplanes, simply because there is available no imaging technology enabling pilots to see the ground in fog. This is obviously highly inconvenient.
There are numerous other applications where it would be highly desirable to have a sensor not affected by these atmospheric constituents. For example, the rescue of an airman forced to eject from his aircraft over water, or of a capsized sailor, is very difficult in fog. Radio frequency beacons can be used to provide a rough position signal, but the wavelengths of such systems are so long that it is impossible to precisely locate an individual beacon with accuracy sufficient to rescue the individual. That is, a radio frequency beacon can be used to provide a "fix", but this is only sufficient to bring a rescue craft within visible range; hence such a system is only effective on clear days.
It is well known of course that microwaves penetrate fog, dust, smoke, etc. generally as do millimeter waves. However, the wavelengths employed by conventional microwave equipment are in general too long to provide accurate imaging using components of practical size. Similarly, microwave sources and detectors are physically so large that microwave equipment is not suitable for numerous airborne and other applications where size and weight are critical.
Thus, while visible and infrared wavelengths are short enough to permit generation of an accurate image of a field of view, radiation in these bands is significantly attenuated by common atmospheric constituents, limiting the utility of such imaging systems. Lower frequency radiation such as radiofrequency energy and microwaves which are not attenuated by these constituents are of such long wavelengths as to be incapable of forming images, or require impractically large components, or both.
Millimeter waves (typically 1 cm-1 mm wavelength, and 30-300 GHz frequency) are between the microwave and infrared radiation bands, and combine some of the advantages of both. Millimeter waves are attenuated by the common atmospheric constituents listed above, but only to a limited degree, such that image formation is possible under circumstances which prevent operation of visible-frequency or infrared systems. Millimeter waves are also of sufficiently short wavelengths that an accurate image of a field of view can be formed using components of sizes convenient to be carried by aircraft, spacecraft, satellites, and so on.
One reason why millimeter wave imaging systems have not heretofore been provided is because prior to the present invention (and those described in the related applications referred to above) there has been provided no teaching of practical components for detecting millimeter wave energy emitted by or reflected from objects in a field of view which are suitable for forming an image.
There are two principal types of array useful for forming images of objects in a field of view. These are staring sensor arrays, consisting of aperture-plane and focal-plane sensor arrays, and scanned-sensor systems. Scanned-sensor systems include conventional television cameras (in which an electron beam is electronically deflected to scan a photosensitive image-forming screen in raster fashion), systems in which a single sensor is mechanically scanned to "sweep" across the field of view (such as conventional radar systems), and phased-array radar systems (in which the phase of a source of energy directed into the field of view is varied, such that the phase of energy reflected from objects in the field of view can be employed to determine their relative positions).
Each of these "scanning-sensor" systems has inherent defects or limitations which prevent or limit its effective employment in millimeter-wave imaging systems. Photosensitive elements responsive to millimeter waves have not been developed, so conventional television camera technology is unavailing. Mechanical scanning involves significant complexity, and limits the rate at which information can be gathered from the field of view. Phased-array techniques also are very complex to implement effectively, especially at millimeter-wave frequencies.
The alternative to a scanned-sensor system is a staring-sensor system, in which discrete imaging elements are provided to form separate picture elements ("pixels") of the image. As mentioned, these fall into two classes: aperture-plane arrays and focal-plane arrays. The Very Large Array radio telescope installed in New Mexico is an example of an aperture-plane array device. The sheer complexity of such instruments prevents their use in aircraft, for example. Furthermore, the signals received by each of the detectors in the array must be extensively computer-processed to yield the image, so that a real-time image cannot be formed.
A focal-plane array is a second type of staring sensor, and that which is employed according to the present invention. As used herein, the term "focal-plane array" refers to an imaging sensor system in which a lens or equivalent focusing element is used to focus the energy from the field of view onto an array of imaging elements, and in which different imaging elements correspond to different portions of the field of view and thus to the corresponding pixels of the ultimate image. An ordinary still camera can be considered a focal plane array imaging system, in that the film is disposed in the focal plane of the lens and directly records the image. Another type of focal plane array is employed in so-called CCD video cameras, in which a planar array of individual charge-coupled-device (CCD) elements, each generating a pixel of the image, is disposed in the focal plane of an optical lens. However, there have not been developed any CCD focal plane arrays or film suitable for imaging millimeter wave radiation.
Proposals have been made for focal-plane arrays of a limited number of sensing elements responsive to millimeter waves. For example, see Yngvesson, "Near-Millimeter Imaging with Integrated Planar Receptors: General Requirements and Constraints", in Infrared and Millimeters Waves, vol. 10, pp. 91-110 (1983); Yngvesson, "Imaging Front-End Systems for Millimeter Waves and Submillimeter Waves", SPIE Conf. on Submillimeter Spectroscopy (1985); and Korzeniowski et al., "Imaging System at 94 GHz using Tapered Slot Antenna Elements", Eighth IEEE Int'l Conf. on Infrared and Millimeter Waves, 1983.
These papers discuss millimeter wave imaging generally, and suggest systems in which energy from the field of view is coupled to an array of detector elements. These detector elements may each consist of a pair of "Vivaldi", i.e. exponentially-tapered, antenna members, with a diode detector coupled thereacross. See FIGS. 7 and 8 of the Infrared and Millimeter Waves paper. Yngvesson suggests in the first paragraph on the second page of the SPIE paper that prime-focus or offset paraboloids or Cassegrain-telescope reflectors are the principal choices for focusing elements. Lenses are discussed as secondary elements on page 4 of this paper.
Yngvesson suggests in the SPIE paper that the local oscillator signal can be injected through a hole in the reflector. This involves very substantial constraints on the design of the system, particularly with respect to uniform illumination of the array. In the Korzeniowsky et al. paper, a lens-imaging system is shown in FIG. 1, but no mention is made of the local oscillator signal.
It can therefore be seen that there is a distinct need in the art for a millimeter wave sensor capable of detecting millimeter wave radiation reflected from or emitted by an object in its field of view and suitable for providing an image signal essentially in real time, that is, without requiring extensive processing, and wherein each pixel of the image signal corresponds to one of the imaging elements of the sensor array. Such, a sensor would have numerous applications throughout numerous fields including transportation, surveillance, and mapping. For example, a millimeter wave imaging-sensor system, optionally employed in combination with millimeter-wave landing lights or runway beacons, could be used to provide a video picture of an airport runway to a pilot attempting to land an airplane in fog. Millimeter wave sensors of this type could also be used to provide an additional level of security at airports and the like, such that objects invisible to x-ray inspection devices currently used could be imaged; such a system could be used to interdict drug shipments and perhaps also for plastic weapon detection. Other uses of the millimeter wave imaging sensor systems are discussed below.